The field of the invention pertains to high-temperature fluid pressure sensors and, in particular, to movable diaphragm fiber-optic pressure sensors.
It is highly desirable that high-temperature pressure sensors demonstrate little or no errors associated with varying or high temperature. In particular, pressure sensors used at very high temperatures such as those encountered under combustion conditions are subject to large temperature-related errors affecting sensor offset and sensitivity accuracy. To minimize such temperature-related errors, the most accurate current sensors use water or air cooling to maintain the lowest possible equilibrium temperature. However, due to physical constraints in vehicle internal combustion engines, water or air cooling is not practical for in-vehicle testing or long term continuous engine monitoring.
A diaphragm type fiber-optic pressure sensor measures pressure by detecting light intensity changes resulting from the pressure induced diaphragm movement with respect to the sensing and delivering optical fibers. In effect, the distance between the diaphragm and the optical fibers is measured and the distance is dependent on both mechanical and optical sensor characteristics. While almost free of errors associated with temperature dependent optical fiber transmission, the sensor is subject to errors due to mechanical property changes and thermal expansion of other sensor components.
This invention further improves the technology disclosed in U.S. Pat. No. 5,600,070 issued to one of the instant applicants and teaches a high-temperature fiber-optic pressure sensor compensated for the effect of varying temperature on sensor performance. Techniques described below are aimed at correcting for the sensor sensitivity and offset dependence on temperature. By using materials of different thermal expansion coefficients for the sensor diaphragm, housing, ferrule and fiber-bonding compound and by optimizing the length of such parts, the relative distance of the fiber tip with respect to the sensing diaphragm changes in a manner that reduces sensor sensitivity and/or offset dependence on temperature.
In the first embodiment, the distance change results predominantly from controlled fiber movement within the ferrule and is used to reduce the temperature sensitivity of dynamic sensors. In the second embodiment an optimum selection of the diaphragm, housing, ferrule and bonding compound materials yields a stable fiber position within the ferrule but, instead, a well defined ferrule movement with respect to the diaphragm in response to temperature changes. The latter technique is used to reduce the offset error of static sensors or the sensitivity error of dynamic sensors.
The offset and sensitivity errors arise from fundamental material responses to temperature change. One error arises from a diaphragm""s deflection increase with temperature via the temperature dependence of the Poisson number and Young""s modulus. The second error is due to fiber tip movement relative to the diaphragm resulting from thermal expansion effects on the sensor structure. Those effects are due to different thermal expansion coefficients and dimensions of such sensor components as the fiber bonding compound, the ferrule, diaphragm and housing. As a consequence, a changing temperature introduces errors in the sensor output signal which may be expressed as
V(p)=V(po)+(S*P) 
and affecting both the sensor offset V(po) and sensitivity S.